"Mark" wrote in message
oups.com...



Must be my lucky week!. I needed to make some similar readings.
Thanks!.
I still puzzle over the oft quoted "up to about 30mVrms the output
from the
diode offers a square law response and will approximate a true RMS
measurement.
As the OP found, there's very little happening down there. Who makes
these
sweeping statements?.
regards
john

No it's true. Look at the curves and notice that the current scale on
the x axis is a log scale. When the RF input is very small, the DC out
is proportional to the log of the RF level i.e. the RF in dB. This is
how the normal power meter works. It also provides a true RMS value
for modulated RF signal. Once the signal gets too big and the diode
begins to work as a converntoin rectifier, this relationship no longer
holds true. Notice the curves break upeards. When the RF volatge to
log I curves are straight line, this is rhe square law region where the
diode current gives you true RMS readings of the RF voltage.
Think of it as a voltage in dB to current converter.

Mark

I know where you're coming in from, but whichever way I look at Win's graphs
I'm seeing a straight line for that 1N4148 at 0-30mv levels. Yes, the scales
are log-log but the constant of proportionality is dead straight linear. I.e
1mv RF in gives 1e-10 amps and 10mV in gives 1e-9 amps and pro-rata for all
points in between (you did see the double decade increments?). This agrees
with the 10Mohm value that's marked on the graph.
It shouldn't matter if the RF signal is swept over a 1 to 30mV range or just
a 5 to 5.01mV range, the DC out will be directly proportional to the 'DC' in
and no distortion of the waveform can occur, hence no dc offsets or
harmonics.
The graph next door though , the 1n5819, looks like it could offer up a tad
of rf dB-I rectification. Though to my eyes it still looks way more like a
resistor than anything with a square or log law response.
There'll be better devices out there that offer (say) quadratic like
classical responses at these low levels but it's moving out of the 'common
or garden' playground.
regards
john